Mr device provided with differently optimized rf coil arrays

ABSTRACT

The present invention relates to an MR device for MR imaging as well as to an RF coil system for such an MR device. In order to enable switching to and fro between different applications in such an MR device without having to move the patient so as to position a new RF coil system, it is proposed in accordance with the invention to provide the RF coil system for the transmission and/or reception of RF signals with at least two RF coil arrays which are integrated in one coil former and have been optimized for different applications, each RF coil array comprising at least two RF coils which are decoupled from one another.

The invention relates to a magnetic resonance (MR) device for MR imagingas well as to an RF coil system for such an MR device.

MR devices of this kind are generally known and described in numerousdocuments, for example, in WO 00/72034 A1 which discloses a magneticresonance device for carrying out the SENSE method by means of an RFcoil array. The optimization of RF coil arrays is of major importancefor MR imaging. For given clinical protocols a specific optimization ofthe image quality can be achieved by parameter variation of the coilnumber, the coil configuration and the arrangement of the coils. Theobject basically consists of obtaining a maximum signal-to-noise ratio(SNR). A high SNR in deeper layers is achieved by means of RF coilshaving a given minimum size. However, the maximum number of RF coils inrelation to the given size of the object to be examined, for example, apatient, is thus limited. When using SENSE or SMASH imaging methods,however, increasing the number of RF coils is an absolute necessity soas to obtain a high reduction factor for a corresponding temporalresolution.

It can be stated in principle that the design criteria for RF coilsdiffer significantly in dependence on the relevant application and theimaging method. For example, when the SENSE imaging method is used, alow error propagation rate should be achieved in combination with a highreduction factor, whereas maximization of the SNR is most important whensynergy coils are used. It follows therefrom that an RF coil array whichhas been optimized for the SENSE method also deviates geometrically froma synergy coil array in respect of the number, the size and the positionof the RF coils.

For clinical applications it is desirable that switching over betweenand selection of different applications and imaging methods can takeplace at will without it being necessary to move the patient in order tofit a new RF coil array. Therefore, it is an object of the invention toprovide an MR device as well as an RF coil system for such an MR devicewhich enable such selection and switching.

This object is achieved in accordance with the invention by means of anMR device as disclosed in claim 1 which includes:

-   -   a main field magnet for generating a steady main magnetic field;    -   a gradient coil system with a plurality of gradient coils for        generating magnetic gradient fields;    -   an RF coil system for transmitting and/or receiving RF signals,        which coil system includes at least two RF coil arrays which are        integrated in one coil former and have been optimized for        different applications, each RF coil array comprising at least        two RF coils decoupled from one another;    -   a transmit/receive unit for driving the RF coil arrays and for        receiving MR signals from the RF coil arrays, there being        provided a plurality of channels, notably a number of channels        which corresponds to the number of RF coils of the RF coil array        comprising the largest number of RF coils;    -   a control unit for controlling the MR imaging, the control unit        being arranged to switch over the RF coil arrays for temporally        separate use of the individual RF coil arrays during the MR data        acquisition; and    -   a processing unit for processing received MR signals.

The simultaneous integration of RF coil arrays optimized for differentapplications in one coil former in accordance with the invention offersmajor advantages. During an examination it is no longer necessary tomove the patient, or even to move the patient to a different bed, whenthe application of a different MR imaging method is desired; the overallexamination time is thus reduced. The different RF coil arrays can nowbe specifically selected separately for given clinical applications. Forexample, when the different RF coil arrays are suitably optimized, anoptimum can be achieved as regards the SNR and/or the highest temporalresolution.

Advantageous embodiments of the MR device in accordance with theinvention are disclosed in the dependent claims. An RF coil system forsuch an MR device is disclosed in claim 10.

In the MR device in accordance with the invention at least the RF coilswithin the individual RF coil arrays are decoupled from one another.Only one RF coil array can thus be used at any time for the excitation(in the transmit mode) or for the acquisition of MR signals (in thereceive mode), so that the RF coils of the other RF coil arrays areelectronically switched off. The switching over between the individualRF coil arrays can be carried out directly from a control console forthe relevant imaging protocol or by the relevant imaging sequenceitself.

In a preferred embodiment it is also arranged that the individual RFcoil arrays are also decoupled from one another, so that the individualRF coils of all different RF coil arrays are also decoupled from oneanother. As a result, in as far as there is provided a correspondinglylarge number of channels in the transmit/receive unit, MR signals can bereceived in parallel from all RF coils. Appropriate switching means maybe provided so as to enable switching over at will between RF coils ofdifferent RF coil arrays in the case where the total number of RF coilsis larger than the total available number of channels of thetransmit/receive unit. This enables the simultaneous application ofdifferent imaging methods during one MR data acquisition; this is ofspecial interest for special applications.

In conformity with a further embodiment of the invention, a first RFcoil array is advantageously optimized for the SENSE method or the SMASHmethod and a second RF coil array is optimized as a synergy coil array.With respect to the SENSE method, reference is made to the publicationby K. Prüssmann “SENSE: Sensitivity Encoding for Fast MRI”, MagneticResonance in Medicine 42: 952-962 (1999) and WO 99/54746 in which thismethod is described in detail. The SMASH method is described in WO98/21600. The RF coil array for SENSE or SMASH methods is then optimizedin order to achieve a reduction of the acquisition time, whereas asynergy coil array is intended to achieve a maximum signal-to-noiseratio.

Preferred further versions of this embodiment are disclosed in theclaims 4 to 6. In conformity therewith it is arranged in particular thatthe RF coils of the SENSE or the SMASH RF coil arrays are situatednearer to the object to be examined, are smaller in size and larger innumber and are arranged so as to overlap one another, as opposed to theRF coils of the synergy coil array which are preferably arranged so asthat they do not overlap one another.

Generally speaking, switching over between the various RF coil arrays inconformity with the clinical protocol can take place after theacquisition of complete sets of image data. However, as is indicated inclaim 7 it may also be arranged that all RF coils are connected to aseparate channel of the transmit/receive unit and that the control unitis arranged for the simultaneous acquisition of MR signals by means ofRF coils of different RF coil arrays. MR signals can thus be acquiredsimultaneously from different regions and with a different destinationdirection, thus enabling advantageous applications. For example, it isfeasible to reconstruct images in real time already during the MR dataacquisition, for example, from MR data acquired by an RF coil arraywhich has been optimized for the SENSE method. Images of this kind thendepict changes of the object to be examined with a high temporalresolution as is of interest, for example, for MR angiography. Suchreal-time data can also be fed back to the data acquisition so as toenable motion correction or control of the data acquisition in general.

It is in principle also possible to switch over the mutually decoupledRF coil arrays within an imaging sequence (switching time approximately100 μs). New methodic protocols can thus be applied, enabling the use ofthe different RF coil arrays for the data acquisition from onlysub-regions of the k space. For example, the data of the central k spacecan be measured with a high SNR, for example, by means of a synergy coilarray, whereas the high k spatial frequencies are acquired at a highspeed, for example, by means of a SENSE RF coil array. The correspondingimages can be acquired by way of a suitable calibration, for example, ofthe SENSE coil array to the synergy coil array, and an adaptedreconstruction. The use of an MR device thus elaborated, as disclosed inclaim 9, enables combination of the advantages of a reduction of themeasuring time and a maximum SNR.

The invention will be described in detail hereinafter with reference todrawings. Therein:

FIG. 1 is a diagrammatic representation of an MR device in accordancewith the invention;

FIG. 2 a shows a first embodiment of an RF coil system in accordancewith the invention;

FIG. 2 b shows an associated switching unit;

FIG. 3 shows a second embodiment of an RF coil system in accordance withthe invention;

FIGS. 4 a, b show a third and a fourth embodiment, respectively, of anRF coil system in accordance with the invention with switching means;

FIG. 5 shows a diagram illustrating an application of the MR device inaccordance with the invention;

FIGS. 6 a to e are various views of a SENSE RF coil array; and

FIGS. 7 a to g show different versions of RF coil arrays in accordancewith the invention.

FIG. 1 is a diagrammatic representation of an MR device in accordancewith the invention for forming MR images of the patient 15 who isarranged on a patient table 19 in the examination zone. The MR deviceincludes a main field magnet system 10 with a plurality of main fieldmagnets which generate a steady, uniform magnetic field in thelongitudinal direction of the patient 15. A gradient coil system with aplurality of gradient coils 11, 12, 13 is provided so as to generatemagnetic gradient fields. Furthermore, an RF coil system 14 is providedto generate RF excitation pulses and to acquire MR signals from theexcited examination zone, the construction of said RF coil system inaccordance with the invention being described in detail hereinafter. Atransmit/receive unit 16 is provided in order to control the individualRF coils of the RF coil system 14 in the transmit mode or the receivemode for the MR signals received by the individual RF coils. The MRsignals received are processed by a processing unit 17 so as to formdesired MR images. Finally, a control unit 18 is provided for thecontrol of the transmit/receive unit 16, the processing unit 17 and thevarious coil systems 10 to 14. Further details of the basic constructionof such an MR device as well as of its operating principle are generallyknown, for example, from the previously mentioned WO 00/72034 and,therefore, will not be elaborated herein.

FIG. 2 a shows a first embodiment of an RF coil system 141 in accordancewith the invention. The Figure shows two RF coil arrays 20, 21 which areformed as surface coils and are arranged one over the other around thepatient 15 who is shown in a cross-sectional view. The RF coil array 20which is nearest to the patient 15 includes a total number of eight RFcoils 201 to 208 which are arranged adjacent one another and withoutoverlapping one another. These coils have been optimized for applicationof the SENSE technique. On these coils there are arranged the four RFcoils 211 to 214 of the second RF coil array 21, that is, in such amanner that each time two RF coils slightly overlap one another and thatall RF coils 201 to 208 of the first RF coil array 20 are covered. TheRF coils 211 to 214 are configured as synergy coils.

Each of the RF coils 201 to 208 and 211 to 214 is connected to arespective preamplifier 22, so that there are twelve connection points Ato L in total. In as far as the transmit/receive unit (16 in FIG. 1)comprises only eight channels, in this case a switching unit 23 as shownin FIG. 2 b can be used to switch the twelve connection points A to Lcorrectly to the eight channels 1 to 8. The switching unit 23 is alsocontrolled by the control unit 18 for this purpose.

FIG. 3 shows a further embodiment of an RF coil system 142 in accordancewith the invention. In addition to the RF coil systems 20 and 21 shownin FIG. 2 a, the RF coil system 142 includes a head volume coil 24,being a so-called birdcage coil, which encloses the two RF coil arrays20, 21. This embodiment is intended in particular for the acquisition ofMR images of the head of a patient. For the head volume coil 24 there isalso provided a separate preamplifier 22 with a separate input M1 (fortransmission) and an output M2.

Two further embodiments of an RF coil system 143, 144 in accordance withthe invention are shown in the FIGS. 4 a, 4 b. Fewer channels arerequired in these embodiments, because appropriate switching means 25and 23 are provided in front of or behind the preamplifiers 22. Each ofthe embodiments 143, 144 shown is provided with two synergy coils 30, 31and three SENSE coils 32, 33, 34, decoupling capacitances C_(K) beingprovided for the decoupling of the RF coils. Alternatively, other meanssuch as, for example, λ/2 leads or transformers may be provided for thedecoupling. The embodiments shown are suitable in particular for cardiacexaminations. It may also be arranged that only a part of the RF coilsystem consists of a combination of two RF coil arrays, whereas anotherpart of the RF coil system constitutes a conventional surface coil.

An advantageous application of an MR device in accordance with theinvention, in which all RF coils of the RF coil system used aredecoupled from one another so that in principle MR signals can bereceived from all RF coils simultaneously, will be described in detailhereinafter with reference to FIG. 5. In this respect it is assumed thata synergy coil array and a SENSE coil array are provided. The MR data 40acquired consists of synergy coil data 41 on the one hand and SENSE coildata 42 on the other hand. The k space as well as the filling of the kspace with the acquired MR data are shown each time symbolically. TheSENSE data 42 can be used for the reconstruction in real time, duringthe data acquisition (of the synergy data 41), of images 43, 44, 45 fromthe k space data sets which are interleaved in different ways andreflect with a high temporal resolution the changes of the object to bemeasured. The real-time data can, moreover, be fed back to the MR dataacquisition (feedback 47), for example, in order to carry out a motioncorrection or a general control of the data acquisition. Finally, aconventional MR image 46 with a high signal-to-noise ratio can also bereconstructed from the synergy coil data 41. It is to be noted that thisprocedure is not limited to the combination of a synergy coil system anda SENSE coil system and that it can in principle be used also in thecase of combination of other RF coil arrays.

RF coil arrays are used in principle to enhance the signal-to-noiseratio. The duration of the image acquisition in principle is notaffected thereby. The previously mentioned SENSE and SMASH methodsreduce the acquisition time for an MR image at the expense of the SNR.In both methods the Field Of View (FOV) is reduced, thus giving rise tobackfolding or aliasing. The images acquired contain aliasing artifactswhich must be corrected again at a later stage. This is done by means ofthe MR data acquired from the individual n RF coils of the RF coilarray, because they “see” one of n pixels differently.

The SENSE method solves a system of equations pixel-by-pixel from theimages of the individual RF coils with different profiles, whereas theSMASH method yields a system of equations for an entire MR image. TheSMASH method utilizes in the simplest case a linear RF coil array of RFcoils and synthesizes a plurality of sinusoidal harmonics from the coilsensitivities. These harmonics produce an offset in the k space like aB0 gradient field. Therefore, hypothetically speaking, thereconstruction time for the SMASH method is shorter than for the SENSEmethod, but this is “achieved” at the expense of the image quality. TheSENSE method offers a better image quality. In principle, therefore, thetwo reconstruction methods can be used for a linear RF coil array.Because of the novel and fast hardware reconstruction units availablenowadays, the reconstruction time no longer poses a problem in respectof temporal resolution in the case of the SENSE method.

Synergy coils serve first of all to produce an optimum signal-to-noiseratio. This, of course, also holds for SENSE coils, but now it is notnecessary to observe the secondary condition as regards a suitablesolution of the system of equations for the reconstruction. Generallyspeaking, clinically a high SNR is required with as few artifacts aspossible, that is, an as high as possible SNR and intensity distributionacross the entire FOV. This is achieved in principle already by means ofa small number of RF coils. The combination of these RF coils so as toform an overall image provides an SNR for the center which could not beincreased significantly further by increasing the number of RF coils.

A synergy coil array of this kind can in principle also be used for theSENSE method. In order to obtain a high temporal resolution, the RFcoils can be subjected to modifications which are less suitable for anoptimum uniform image quality. On the one hand the individual SENSEcoils are not arranged so as to overlap one another, but are situated ata given distance from one another, for example, in conformity with FIG.6 a which shows three SENSE coils 50, 51, 52 which are situated at adistance d of, for example, from 5 to 10 mm from one another. As isshown in FIG. 6 b, decoupling capacitances C_(K) are provided for thedecoupling of the three coils 50, 51, 52, whereas appropriate resonancecapacitances C_(T) are provided for adjustment of the resonance. Twofurther embodiments of a SENSE RF coil array with capacitive decouplingand inductive decoupling by means of local RF transformers T,respectively, are shown in the FIGS. 6 c and d. The RF coils may also bearranged so as to be tilted relative to one another as is shown on thebasis of five SENSE coils 60 to 64 in FIG. 6 e. Moreover, the number ofRF coils should be as large as possible.

The FIGS. 7 a to 7 g are diagrammatic representations of furtherembodiments of an RF coil system in accordance with the invention. TheFIGS. 7 a and 7 b both show four synergy coils 70 to 73 with arespective preamplifier 22. Each of the coils 70 to 73 is decoupled bymeans of decoupling capacitances C_(K), but connected together in adifferent way.

The FIGS. 7 c and 7 d show two further synergy coil arrays with eachtime four synergy coils 70 to 73. The decoupling capacitors C_(K)therein are arranged each time at the center. For tuning there each timea different number of tuning capacitors C_(T) is provided again.

FIG. 7 e shows a further embodiment of a SENSE RF coil array. This arraycomprises six coil pairs 80 to 85, each of which comprises two RF coils,which are decoupled via a decoupling capacitance C_(K), and also eachtime two preamplifiers. The coil pairs 80 to 85 are decoupled from oneanother via the distance and the high-ohmic input resistance of thepreamplifiers 22.

FIG. 7 f shows a combination of a SENSE RF coil array as shown in FIG. 7e and a synergy coil array. The eight coil pairs 80 to 85 are coveredsubstantially completely by the synergy coil array which consists of twosynergy coils 90, 91. Of course, other SENSE coils as well as adifferent number and arrangement of SENSE coils or synergy coils canalso be coupled to one another.

FIG. 7 g shows an alternative SENSE or SMASH RF coil array with fourSENSE RF coils 100, 102, 102, 103. Such an RF coil array is preferablycombined with a synergy coil in the form of a large loop coil.

In accordance with the invention at least two RF coil arrays which havebeen optimized for different applications are integrated in one coilformer. The construction of such combined RF coil arrays may, forexample, take the form of a sandwich. Various solutions are feasible inrespect of the number, the arrangement and the configuration of theindividual RF coils or the RF coil arrays, so that various novel MRimaging methods become feasible. Overall the MR device in accordancewith the invention offers a substantial reduction of the dataacquisition time and enhances the ease of operation at the same time.

1. An MR device for MR imaging, which device includes: a main fieldmagnet for generating a steady main magnetic field; a gradient coilsystem with a plurality of gradient coils for generating magneticgradient fields; an RF coil system for transmitting and/or receiving RFsignals, which coil system includes at least two RF coil arrays whichare integrated in one coil former and have been optimized for differentapplications, each RF coil array comprising at least two RF coilsdecoupled from one another; a transmit/receive unit for driving the RFcoil arrays and for receiving MR signals from the RF coil arrays, therebeing provided a plurality of channels, notably a number of channelswhich corresponds to the number of RF coils of the RF coil arraycomprising the largest number of RF coils; a control unit forcontrolling the MR imaging, the control unit being arranged to switchover the RF coil arrays for temporally separate use of the individual RFcoil arrays during the MR data acquisition; and a processing unit forprocessing received MR signals.
 2. An MR device as claimed in claim 1,wherein the at least two RF coil arrays are decoupled from one another.3. An MR device as claimed in claim 1, wherein a first RF coil array hasbeen optimized for the SENSE method or the SMASH method and a second RFcoil array has been optimized as a synergy coil array.
 4. An MR deviceas claimed in claim 3, wherein the RF coils of the SENSE RF coil arrayor the SMASH RF coil array are arranged in the coil former in such amanner that they are situated nearer to the object to be examined thanthe RF coils of the synergy coil array.
 5. An MR device as claimed inclaim 3, wherein the SENSE RF coil array or the SMASH RF coil arraycomprises more and smaller RF coils than the synergy coil array.
 6. AnMR device as claimed in claim 3, wherein the RF coils of the synergycoil array are arranged so as to overlap one another and that the RFcoils of the SENSE RF coil array or the SMASH RF coil array are arrangedso that they do not overlap one another.
 7. An MR device as claimed inclaim 1, wherein all RF coils are connected to a separate channel of thetransmit/receive unit and that the control unit is arranged for thesimultaneous acquisition of MR signals by means of RF coils of differentRF coil arrays.
 8. An MR device as claimed in claim 7, wherein thereprovided means for feeding back MR signals acquired and evaluated inreal time to the control unit so as to change the control of theinstantaneous MR data acquisition in conformity with the MR signalsacquired and evaluated in real time.
 9. An MR device as claimed in claim7, wherein the control unit is arranged to acquire MR signals from afirst sub-region of the k space by means of a first RF coil array,notably for the acquisition of MR signals from the central region of thek space by means of a synergy coil array, and to acquire MR signals froma second sub-region of the k space by means of a second RF coil array,notably for the acquisition of MR signals from edge regions of the kspace by means of a SENSE RF coil array or a SMASH RF coil array.
 10. AnRF coil system for an MR device as claimed in claim 1 for thetransmission and/or reception of RF signals, comprising at least two RFcoil arrays which are integrated in one coil former and have beenoptimized for different applications, each RF coil array comprising atleast two RF coils which are decoupled from one another.